Ink concentration of an ink cell, used in optical concentration assemblies, affects the ink jet printing process. As the ink concentration varies, the optimal parameters used to control the ink jet printing process change as a function of the ink concentration.
In a continuous ink jet fluid system, the ink used, which includes a carrier fluid, such as water or a solvent, and dye, is continuously recirculated through the system under vacuum and mixed with air. Evaporation of the carrier fluid due to the air-ink interaction increases the dye concentration of the ink.
Proper dye concentration is essential to the operation of an ink jet print head. The measurement of dye concentration is used to determine the amount of replenisher needed to mix with the ink to compensate for the carrier fluid lost due to evaporation. When printing rates are high, the amount of dye and carrier fluid removed from the system are typically approximately equal and the ink concentration is maintained, thus, only ink is added to the system.
Alternatively, when little or no printing is being done, the system is in an idle condition and the evaporation rate of the carrier fluid is typically higher than the amount of dye removed during printing. In this instance, then, the dye concentration level increases. A replenishment fluid is needed to bring the ink concentration level down to the proper mixture since high ink concentration affects properties of the ink which are critical to the functions of an ink jet print head. As would be obvious to one skilled in the art, affecting ink properties such as viscosity is detrimental, since the energy required to stimulate filaments is determined partially by the viscosity of the fluid.
Maintaining ink concentration in a continuous ink jet printing system is known in the art. In such a system, ink is passed through a manufactured cell that has a gap of approximately 0.1 mm. An LED light source is used to pass light through the gap and two collectors receive the light. One collector is located so as to receive the light directly to be used as a reference, while the other collector is located on the opposite side of the cell and measures the amount of light transmitted through the cell. As the concentration of the ink in the fluid system increases, the light transmitted through the cell decreases, allowing for a measurable difference. This measurable difference can be used to determine the amount of replenisher fluid needed to replace lost fluid in the system. A device for measuring this difference is an optical concentration apparatus (OCA).
Unfortunately, the OCA is limited by the operating wavelength of the LED light source and by the gap of the manufactured cell. Additionally, the operation of the OCA is restricted to solvent based inks that transmit sufficient light in the visible spectrum with the given gap of 0.1 mm in the manufactured cell. Solvent and water based inks with high absorption in the visible spectrum will not operate with an OCA, making the currently used, commercially available cells both expensive and incapable of use with all ink types.
It is seen then that there exists a need for an ink concentration measuring circuit which overcomes the restrictions typically imposed by use of an OCA.